//https://leetcode.cn/submissions/detail/567132741/
int climbStairs(int n) {
    if(n==1||n==2)
    return n;
    int i=1,j=2,sum;
    int count=n-2;
    while(count--)
    {
        sum=i+j;
        i=j;j=sum;
    }
    return sum;
}
